This invention relates to a vapor phase process for catalytically converting one or more lower molecular weight alcohols, optionally in combination with a lower molecular weight aldehyde and/or ether, to a mixture containing at least one higher molecular weight alcohol over an alkaline catalyst and, more particularly, to a vapor phase, continuous process for converting a C.sub.2 or higher alcohol, which may be in combination with one or more additional C.sub.1 or higher alcohols, and optionally, an aldehyde and/or ether to a mixture containing at least one higher molecular weight alcohol over a catalyst which is essentially magnesium oxide.
In recent years there has been an upsurge in interest in the production of both chemicals and transportation fuels from non-petroleum carbon sources such as methane, tar sands, oil shale and the like. This interest has focused for lack of good direct conversion processes on indirect processes, which often go through a synthesis gas intermediate with subsequent conversion of the synthesis gas (Co and H.sub.2) via Fisher-Tropsch and related processes to hydrocarbons and/or oxygenates. Oxygenates, particularly lower alcohols, are common products of such synthesis gas reactions, and high conversion, selective processes to convert an alcohol or a mixture of alcohols to higher molecular weight alcohols have substantial commercial potential.
One potential process uses the well-known, non-catalytic Guerbet reaction which converts a lower molecular weight alcohol to a branched or linear higher molecular weight alcohol in the presence of an alkali metal alkoxide dissolved in the alcohol to be converted. Such processes are uncatalysed, moderate temperature batch reactions. When considered for industrial use, however, the Guerbet reaction suffers an economic disadvantage in that a portion of the starting alcohol (and possibly some of the product) is consumed by oxidation to the corresponding carboxylic acid unless special agents are added. One publication suggests the use of a mixture of potassium hydroxide and boric oxide to suppress acid formation which is said to improve the yield.
More recently, an improved Guerbet reaction has been reported which uses a "catalyst" system employing magnesium oxide, potassium carbonate, and copper chromite for converting, for example, ethanol to higher alcohols including 1-butanol, and 1-butanol to higher alcohols including 2-ethyl-1-hexanol (J. Org. Chem. 22, 540-2 (1957)). The reaction is of the batch type and the "catalyst" is said to have limited lifetime.
Another improvement in the Guerbet reaction, discussed in J. Mol. Catalysis 33, 15-21 (1985), uses a sodium alkoxide mixed with 5% rhodium on alumina as a "catalyst." A mixture of 1-butanol and methanol is said to be converted by the "catalyst" to a mixture of 2-ethyl-1-hexanol and 2-methyl-1-butanol.
Still other batch Guerbet reaction variations include water removal to improve yield and the use of an alkali metal hydroxide "catalyst" (U.S. Pat. No. 3,328,470), the use of an alkali metal alcoholate/boric acid ester "catalyst" (U.S. Pat. No. 2,861,110), and the addition of a nickel "catalyst" to the metal alkoxide (J. Am. Chem. Soc. 76, 52 (1953).
Octane demand has soared in recent years and the growth is likely to continue in the United States. For example, it has been estimated that clear pool octane demand has been increasing by 0.15 units/year in recent years. The addition of alcohols and ethers such as methanol, ethanol and methyl t-butyl ether to gasoline to improve octane number and/or improve the effect of gasoline combustion in internal combustion engines on the environment has been the subject of a number of recent publications.
Methanol is generally made from synthesis gas and ethanol can be made by cabonylation of methanol or more usually from agricultural products by fermentation. Higher alcohols can also result from the catalyzed conversion of synthesis gas. Methanol, while effective if used essentially pure for transportation fuel, is not a good additive for gasoline. Ethanol has shown promise as a gasoline additive, but isobutanol in particular is valuable as it can be dehydrated to isobutylene and reacted with methanol to form methyl t-butyl ether (MTBE) which is an excellent octane improver that can be easily blended into gasoline. Isobutanol is also an effective octane improver. The methyl ether of isopentanol (TAME) is also an excellent octane improver for gasoline. U.K. Patent Application GB 2,123,411 describes a process for making a mixture of octane improving ethers by synthesizing an alcohol mixture containing methanol, ethanol, and higher alcohols and dehydrating the higher alcohols and etherification.
Because of the large amount of methanol available and its problems as a gasoline additive, processes which convert methanol to effective gasoline additives are valuable. Well-known is the Mobil process for converting methanol to gasoline-range hydrocarbons over an aluminum-containing molecular sieve. Little work has been reported on effectively converting methanol to higher alcohols, in particular, isobutanol.
Now a material has been found which allows a continuous, vapor phase, catalytic Guerbet-type of condensation to be effected on a large variety of different alcohols, aldehydes and ethers and their mixtures. In particular, a catalyst effective in continuously converting a mixture of alcohols such as methanol and ethanol or a mixture of methanol, formaldehyde, and ethanol in a continuous vapor phase process to higher alcohols has been found which can produce a substantial percentage of isobutanol in the product. Such a catalyst allows the production of MTBE using exclusively synthesis gas as the source of carbon to the process.